1. Field of the Invention
The present invention generally relates to high voltage power supplies for battery-powered portable systems, and more particularly to a method and apparatus for using slope-based primary feedback to control the off-time of a power supply.
2. Description of the Related Art
Conventional flyback-type power supplies have included a main switching device which is coupled to a source of electrical power and to a primary winding of a transformer. A secondary winding of the transformer has been coupled to a filter circuit and a load. Such flyback-type power supplies have utilized a transformer to provide electrical isolation between a source and a load as well as to store and transfer energy from the primary winding to the secondary winding. Flyback power supplies have typically been used for low voltage applications.
Flyback-type power supplies have experienced a common issue of secondary winding saturation. Secondary winding saturation occurs when the off-time of the primary winding is insufficient to allow complete dissipation of the energy in the secondary winding. Another issue with conventional flyback-type power supplies has been primary leakage inductance. Primary leakage inductance causes the primary winding voltage of the transformer to continue to increase beyond desired levels at transitions of the main switching device.
Fly-back type power supplies have typically utilized pulse width modulation at fixed frequency or fixed on-time as regulators. These regulators have typically included operational amplifiers and comparators.
Flyback-type power supplies have relied on either of two types of off-time feedback. Off-time with flyback-type power supplies has been determined by the particular load, battery voltage, and main switching device of the particular power supply. With one approach, off-time feedback is based on a comparison between an output voltage and a specified reference voltage. Another approach has been to base off-time on a predetermined timeout period (fixed frequency pulse width modulator). Other disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with techniques described herein.